The present invention relates to digital image coding of video signals.
The immense popularity of compact digital audio discs can be partially attributed to the high sound quality and freedom from deterioration of the recording due to aging or repeated playings or from environmental factors such as dust. It is possible to enjoy similar advantages from the digital encoding of video signals as well. However, real time coding and decoding of a video signal requires much more signal bandwidth than does an audio signal, currently available transmitting and recording media provides insufficient storage capacity for video signals. It is necessary, therefore, to use some form of signal compression or digital bit reduction technique in order to record digitally-encoded video. A number of such bit reduction or signal coding methods, that will enable the recording of full-motion digital images, have been proposed in recent years.
Because the available data rate in present transmission channels and storage media is limited, these known coding methods act to reduce the data rate of the video signal. For the purpose of storing digitally-encoded full motion images on optical storage media, such as CD-ROM or MOD (Magneto Optical Disc), for example, a maximum video data rate of about 1.2 M bits/second is allowed.
One example of a system for digitally coding and decoding video signals is disclosed in an article entitled "Scene Adaptive Coder", W. H. Chen, W. K. Pratt, IEEE Transactions on Communication, Vol. COM-32, No. 3, issued in March, 1984 in which scene adaptive coding/decoding of video signals is described. The described coding technique utilizes discrete cosine transform (DCT), a variable threshold for the DCT coefficients, quantization techniques, Huffman coding and a buffer in the coder. The decoder includes a buffer, Huffman decoder, threshold adding and inverse DCT.
Another such coding/decoding system is disclosed in "Comparison Between Progressive and Interlaced Scanning for a Future HDTV System with Digital Data Rate Reduction", D. Westerkamp, H. Peters, Proceedings of the 2nd International Workshop of Signal Processing for HDTV, l'Aquila, Feb. 29-Mar. 2, 1988. The described system contains a motion adaptive intrafield/interframe coding processor, in which the digitized video data is transformed by means of DCT to obtain the quantizing coefficients.
In the previously described systems the fullness of the buffer controls the threshold and the quantizer step size for the DCT coefficients. Both systems guarantee that the buffer will have no overflow or underflow within a given frame. This causes a varying quantizer step size and a consequently varying image quality within each frame.
In U.S. Pat. No. 4,785,349, different regions of a video frame are coded in a different manner. This may also cause a varying image quality within each frame.
From the description of the previously described systems, it is known that, for a given coding scheme, the more a block of video data is quantized, the less bits are required for coding. It is also known that the quantization level may be determined by the image, or video signal, content.
Two currently known methods may be used to compute the level of quantization required to compress the data rate of a video signal within an available number of bits N. The first method computes a threshold/quantization step size so that a frame can be coded in approximately N bits. The quantizer is then applied with this uniform step size to the whole frame. The second method scans blocks of video data by rows and columns in order to compute a local threshold/quantization step size.
The first method allows a spatially uniform quantization step size to be applied to the whole image. However, computing the right level of quantization in order to code the image to as close as possible to N bits is very difficult. The second method allows the computation of a quantization step size on a block by block basis. However, this method does not provide a uniform level of quantization in the spatial domain; therefore the image quality may be different for the blocks of each frame.